The present invention relates to an imaging device which works utilizing acoustic energy, and particularly to an acoustic microscope.
In recent years, attention has been given in the medical world to ultrasonic waves that can be effectively utilized for observing the internal structure of human bodies. Namely, ultrasonic waves have a property to penetrate through materials that may be optically opaque to light or electron rays. The higher the frequency, the more finely the objects can be described. Furthermore, the data obtained with ultrasonic waves reflect dynamic properties of the objects, such as elasticity, density, viscosity, and the like, and make it possible to learn the internal structure that could not be obtained with light or electron rays.
Study has been forwarded concerning the acoustic microscope which makes the most of ultrasonic waves by utilizing ultra-high frequency sound waves of as high as 1 GHz, i.e., having a sound wavelength of about 1 .mu.m in the water (literature entitled "A Scanning Acoustic Microscope" by R. A. (Lemons) and C. F. (Quate), IEEE Cat. No. 73 CH 14829 SU, pp. 423-426).
The principle of an acoustic microscope consists of mechanically scanning the surface of a specimen in a two-dimensional manner with an acoustic beam which is focused to as narrow as about 1 .mu.m, collecting the disturbed sound waves such as those scattered and reflected by the specimen or those attenuated as they travel through the specimen, converting the collected sound waves into electric signals, and displaying the electric signals on a cathode-ray tube in a two-dimensional manner in synchronism with the mechanical scanning, thereby to obtain a microscope image.
If the sound waves which have transmitted through the specimen are detected and displayed on the acoustic microscope, the obtained image reflects the distribution of acoustic attenuation constant (hereinafter simply referred to as attenuation constant) of the specimen. In the practically used apparatus, the intensity of RF pulses for oscillating the sound waves is fixed, and the amplification factor of an amplifier which amplifies sound wave detection signals is suitably adjusted such that the image is displayed on the cathode-ray tube with a suitable brightness. According to the conventional apparatus, therefore, there exists no definite relation between the attenuation constants of the specimen and the brightness of signal on the cathode-ray tube. Namely, it is not allowed to use density informations of the obtained sound wave image as measured data of attenuation constant of the specimen.
If mentioned in further detail, even if the amplification factor of the amplifier is displayed, it is difficult to correctly measure the attenuation constant of the specimen. This is because, the transmitting efficiency of a transducer which generates sound waves varies depending upon the frequency. Besides, even if a fixed frequency is used, the abovementioned efficiency varies with aging. Accordingly, to presume the intensity of sound waves incident upon the specimen relying upon the intensity of RF pulses, involves incorrect factors. Another reason is that the sensitivity of a receiving transducer for detecting sound waves that have transmitted through the specimen, also varies depending upon the frequency and aging. Therefore, to presume the intensity of sound waves that have transmitted through the specimen relying the amplification factor of an amplifier which amplifies detection signals or relying upon the brightness of a picture on the cathode-ray tube, also involves incorrect factors.
The present invention deals with an acoustic microscope of the reflection type which obtains a picture that reflects the distribution of attenuation factors of a specimen by detecting echoes reflected from the back surface of the specimen. The acoustic microscope of this type has been disclosed in Japanese Patent Application No. 35828/ 1983 filed on March 7, 1983 that is earlier than the filing date of the present application.